Frowning when sad actually makes you feel more unhappy, research into the beauty treatment Botox suggests.

The anti-wrinkle drug can make people feel better because it stops them frowning when they are unhappy which feeds back to the brain reducing the intensity of the feeling.

The theory is that if they cannot physically frown then the brain feels there may be less to frown about, scientists claim.

They said it is the psychological equivalent of the old song "when you're smiling, the whole world smiles with you" – or when you don't frown, then the world is less sad.

It applies even if the reason for seeming happy is an injection of chemicals into the forehead to stop wrinkles, said the study by the US Association for Psychological Science.

Many celebrities have the shiny forehead and startled rabbit look that suggests they have had Botox treatments, though not all admit it.

Botox injections are a temporary measure in which tiny amounts of the toxin paralyse muscles, particularly those that cause frowning.

Researchers from the University of Wisconsin tested 40 volunteers who had small doses of Botox into their forehead.

They were then asked to read out a series of written statements ranging from ones that were "angry" to "sad" to "happy" both before and after their treatment.

Like any kind of paralysis, blocking the body's natural movement can have an effect on emotion, the study's authors told the journal Psychological Science.

When those who had received the treatment read out the more negative statements they took slightly longer to do so than they had before having the injections.

According to researcher David Havas, the time delay was tiny but significant because it suggests the brain takes longer to process the emotion behind the statements.

Mr Havas said: "There is a long-standing idea in psychology called the facial feedback hypothesis.

"Essentially, it says, when you're smiling, the whole world smiles with you. It's an old song, but it's right.

"Actually, this study suggests the opposite: When you're not frowning, the world seems less angry and less sad."

Research leader Professor Arthur Glenberg added: "Normally, the brain would be sending signals to the periphery to frown, and the extent of the frown would be sent back to the brain.

"But here, that loop is disrupted, and the intensity of the emotion and of our ability to understand it when embodied in language is disrupted.

However you have to be careful, research suggests. Having Botox in the lower part of your face can prevent a smile and have the opposite effect, research last month from Barnard College, New York, suggested.